FIGS. 9-14 illustrate typical designs of an optical encoder system 5 using an emitter/detector module 10 configured to provide closed-loop feedback to a motor control system. There are various configurations of optical encoder system 5 to provide closed-loop feedback to a motor control system. Optical encoder 5 can be designed in a transmissive configuration 15 (FIG. 9), a reflective configuration 20 (FIG. 10), or an imaging configuration 25 (FIG. 11). When operated in conjunction with a pattern medium 30, which typically includes a codewheel 35 (FIG. 12) or a codestrip 40 (FIG. 13), optical encoder 5 translates rotary motion or linear motion into a two-channel digital output.
Referring to FIGS. 9-11, optical encoder 5 contains an emitter 45, such as an LED 45, as a light source. In transmissive encoder 15 (FIG. 9), light 50 is collimated into a parallel beam by means of a lens 55A located over LED 45. Opposite to emitter 45 is a detector 60 that includes photo-diode arrays 65 (FIGS. 12 and 13) and a signal processor (not shown). When pattern medium 30 moves between emitter 45 and detector 60, light beam 50 is interrupted by the pattern of bars 70 and spaces 75 on pattern medium 30.
Similarly, in reflective encoder 20 (FIG. 10) or imaging encoder 25 (FIG. 11), lens 55B (FIG. 10) or another light shaping device over LED 45 focuses light 50 onto pattern medium 30. Depending on the position of pattern medium 30, light 50 is either reflected or not reflected back to emitter/detector module 10. Reflected light 50 may be received through emitter/detector module 10 through lens 80 over photo-detector 60. The movement of pattern medium 30 causes an alternating pattern of light and dark corresponding to the pattern of bars 70 and spaces 75 to fall upon photodiodes 60. Photodiodes 60 detect these interruptions and the outputs are processed by the signal processor to produce digital waveforms. These encoder outputs can be used to provide information about position, velocity and acceleration of the motor.
Referring to FIGS. 12 and 13, light shaping components 55C, 55D, and 55E may be used in addition to or in place of lens 55A (FIG. 9) or lens 55B (FIG. 10). Light shaping component 55C may include a beam expander. Light shaping component 55D may include a collimating lens. Light shaping component 55E may include a cylindrical lens.
Referring now to FIG. 12, there is shown a conventional approach for an optical encoder system 5 using an absolute encoder pattern medium 30. Optical encoder system 5 includes a glass disc 35 with a suitable number of concentric tracks 85, each having alternated transparent zones 70 and opaque zones 75. These patterns allow the angular position of disc 35 to be determined. All tracks are illuminated by LED 45 from one side of disc 35. Light beam 50 is modulated by tracks 85. A narrow radial slit 90 limits the width of light beam 50 passing through. Narrow radial slit 90 is also referred to as collimating slit 90. Collimated light beam 50 is collected by a photodiode array 65 on the side of disc 35 opposite to LED 45. Optionally, a second narrow radial slit 92 is disposed prior to photodiode array 65 so as to reduce cross talk between adjacent detectors and to add flexibility to use the same detector with different levels of resolution by changing the reticle opening. Each photodiode of array 65 is the input element of a read-out channel. Each photodiode of array 65 amplifies the corresponding signal and generates digital waveforms suitable for subsequent processing.
An absolute optical rotary encoder 5 using this conventional approach provides direct and absolute information related to the angular position of a mechanical system to which disc 35 is connected. For example, a drive shaft is one such type of mechanical system. The number of resolution bits (Sn) is equal to the number of disc tracks 85. Typically, gray code is used to minimize read-out errors.
Referring to FIG. 13, there is shown a linear code strip 40, which has an assembly with LED 45 and photodiodes assembly 65 similar to rotary codewheel 35. One difference from rotary codewheel 35 is that linear code strip 40 has a linearized pattern 95 rather than concentric tracks 85. Typically, to obtain higher resolution or accuracy of the encoder, whether rotary codewheel 35 or linear codestrip 40, requires more tracks 85 or traces 95 on pattern medium 30. For example, a 13 bit absolute encoder typically has 13 tracks 85 or traces 95. As each one of tracks 85 or traces 95 occupies space, i.e. radially for rotary codestrip 35 and transversely for linear codestrip 40, the pattern medium 30 essentially defines the cross-sectional width of encoder system 5. Any increase in the width of pattern medium 30 in turn increases the size of the final product containing encoder system 5. It should be understood that 13-bit absolute encoder normally requires 13 tracks on the codewheel. However, there are methods known in the art to achieve higher number of bits than the actual number of tracks on the codewheel through electronic interpolation of the output signals.